Only somebody familiar with it could tell you whether it is normal or not. If you fill in your profile to say where you are, you might find somebody willing to take a listen, who has a D2. It is an annoying noise though, and it triggers far too often when not needed, and then when it is needed it doesn't do the business. I'd take the old fashioned centre difflock any day, but as I use the Defender off road it isn't a big problem for me.
Fitting the centre diff lock kit from Ashcrofts would certainly cut down the racket off road, if you use it a lot off road. I'd also point out that the standard factory tyres are carp, and the ETC has to go all the time with them to stand any chance of getting anywhere. Decent tyres which actaually have some grip are a worthwhile upgrade...